A brake system of this type has been disclosed, for example, by German Offenlengungsschrift DE-OS 36 27 000.
The brake system comprises a master brake cylinder to which wheel brakes are connected through brake conduits. Associated with the wheel brakes are inlet and outlet valves, with the inlet valves controlling pressure fluid flow through the brake conduits and the outlet valves controlling pressure fluid flow through the connection of the wheel brakes to a pressure fluid reservoir. Moreover, pumps are provided that deliver pressure fluid from the reservoir to the working chambers of the master brake cylinder.
The master brake cylinder is provided with central valves controlling the communication between the working chambers of the master brake cylinder and the pressure fluid reservoir. As long as the master brake cylinder is inoperative, the central valves are open so that the brake circuits connected to the working chambers are in communication with the pressure fluid reservoir. A pressure and volume balance can take place at any time. Upon actuation of the brake pedal, the central valves are closed so that the brake circuits are hydraulically isolated from the reservoir, thereby enabling pressure to build up in the brake circuits.
However, the central valves, in addition, perform a control function.
During an anti-locking control operation, the pumps deliver pressure fluid from the reservoir into the master brake cylinder, with the working pistons of the master brake cylinder being restored until the central valves open. Any excessive pressure fluid that cannot be accommodated by the brake circuits is passed to the reservoir, with the central valves operating as control valves adjusting the pressure in the brake circuits in proportion to the pedal force. During this control operation, the central valves open and close in rapid sequence. Such valves are, therefore, required to be of a special design.
Central valves are known in the art wherein at least one element (either the valve body or valve seat) is made of a soft, elastic material. However, central valves of this type will be used only to hydraulically block the brake circuits upon commencement of a deceleration operation. They are not suitable to be used as control valves as the soft, elastic material (e.g. rubber), during closing, would be forced by the volume flow into the opening to be closed and sheared through the pressure increase in the working chamber between the stop face of the valve and the sealing face.
However, valves of this type are relatively insensitive to contamination, as dirt particles in the hydraulic fluid, which, although forced into the rubber, are hermetically enclosed by the rubber so that the closing operation is not affected thereby. Moreover, such valves exhibit a relatively high resistance to wear and damage of the sealing geometry with no functional loss.
When such valves are used as control valves, it is necessary for the valve body and valve seat to be made of a hard material such as steel. However, such materials have the disadvantage that the dirt particles affect or destroy the surfaces of valve body and valve seat, preventing the valve from closing completely. The valves will, therefore, have to be provided with filters for filtering the dirt particles out of the hydraulic fluid.
Filters of that type are difficult to mount. They require large mounting spaces and are made of expensive material.